Optical fiber has long been recognized as a useful medium for communication signals. With the growth of traffic demand on optical networks, and with the advance of communication technologies such as free-space optical communication, there has been a need for optical fiber devices to handle signals at ever greater levels of optical power.
With increasing power, however, comes increasing danger that unconfined light will escape from the fiber and heat materials and components beyond their damage thresholds. This may occur, for example, when light that is nominally end-coupled into a propagating core mode of an optical fiber is partially injected, as stray light, into the fiber cladding. At an optical discontinuity as might occur, for example, where a pigtail fiber emerges from a device package, the stray light can escape from the cladding into the surrounding material, where it is absorbed to create a potentially destructive hot spot.
For example, FIG. 1 shows an illustrative optical assembly in which housing 10 contains a diode laser 20 powered through electrical connector 30. Laser 20 emits light into refractive optics 40 (shown symbolically in the figure as a convex lens), from which the light is focused onto an end of optical fiber 50. More specifically, the light is focused onto a spot that is sufficiently small, and with a sufficiently small entrance angle into the fiber, that most of the light is captured by the fiber core, where it excites one or more propagating core modes of the fiber.
It should be noted in this regard that the representation of a laser diode is merely illustrative and not limiting. Other optical devices in regard to which the invention may usefully be employed include, without limitation, switches, attenuators, modulators, time-delay buffers, filters, polarizers, isolators, circulators, couplers, wavelength demodulators, taps, and amplifiers.
With further reference to FIG. 1, a ferrule 60 is typically used to facilitate the positioning and alignment of fiber 50. As seen in the figure, ferrule 60 passes through a wall of housing 10, so that one end face is situated within the housing, and the other end face is situated outside the housing. The end face of fiber 50 is typically flush with the inward face of the ferrule, and the fiber passes through the outward face of the ferrule and extends further downstream.
Also shown in FIG. 1 is a cladding mode optical power stripper 70, sometimes also referred to as a light dump. The purpose of stripper 70 is to remove stray light that has been inadvertently injected into propagating cladding modes due to misalignment, mechanical perturbations, and the like. Typically, strippers of this kind will be used when at least the inner polymer coating of a fiber is a low index coating. (In fibers with high-index coatings, by contrast, the stray light tends to couple out of the cladding modes into the polymer coating, where it is absorbed.) Low-index coatings are typically encountered in cladding pumped amplifier fibers and in the passive fibers that lead from pump-light couplers to such amplifier fibers. (An alternative, most useful when only a few watts of stray optical power have been coupled into cladding modes, is to strip a short section of fiber, recoat it with a high-index polymer, and add a heat sink.) Optical power strippers are commercially available, and their use is well known.
As noted above, the location where a pigtail fiber emerges from an optical device package is particularly subject to thermal damage due to stray light. Such a location is indicated in FIG. 1 by circle 80. As will be understood from the figure, power stripper 70 is situated some distance downstream, and therefore is unable to protect the optical fiber from damage in the region indicated by circle 80.